On 6/24/2012 9:52 AM, Jim Nichols wrote:
> Does that mean I have to expose at f/5.6 to avoid refraction problems when
> fully extended?
Not necessarily. While all the numbers and equations the various Drs. are
throwing around are accurate for thin (i.e.
single element) lenses, they are only useful as guidelines or starting points
for the real world. They don't address two
important factors.
1. Complex lenses have different DOF characteristics, so the trade-off of
resolution in the focal plane and DOF may vary
by lens and diffraction effects may be more or less obvious at any given
aperture.
2. Many contemporary lenses, going back to some Zuikos and others with moving
elements for close focus back at least 30
years, mess all this up. They change focal length as they focus closer. When
your 100 mm macro lens becomes 60 mm at
1:2, you need to rework the equations. But - nobody publishes this data on
their lenses.
The upshot is simple. Put up a 3D subject that approximates the things you are
shooting. Set up with tripod, focus
carefully, in your case with E-510 live view, and shoot a small series of shots
at various focal distances you use at
apertures from say f4 to the smallest the lens will do. Remember to check focus
for each shot, as many lenses change
focus at least slightly with aperture.
It immediately becomes apparent where the sweet spots are.
I have done this, and the practical results don't always agree with the theory.
Another thing to remember is that the 'theory' isn't pure math/optics. It
started based on studies of human visual
acuity. They showed lots of B&W prints to lots of people to determine at what
point, for the average person, viewing a
specific print size at a specific distance, differences in sharpness could be
seen. The size of the Airy disks chosen
for all the fancy calculations are all based on this type of research. If it
had all been based on my vision, the
numbers would all be different. :-)
We are now viewing in color, sometimes in a web image, sometimes at 100%, often
images made with significantly more
complex lenses. At 100%, diffraction effects are going to be easier to notice.
At 800-1000 pixels wide on a computer
monitor, they will be less noticeable.
Test It Moose
--
What if the Hokey Pokey *IS* what it's all about?
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